Recent years have seen an increase in the number of motor-driven vehicles including electric vehicles, hybrid automobiles, and other environmentally friendly vehicles. Further, in addition to environmentally friendly vehicles, electric four wheel driving vehicles having an engine for directly driving front wheels and a motor for driving rear wheels are also becoming widely used. As a drive motor for these electric automobiles, a synchronous motor is mainly used because of its compact size and high efficiency. There are several types of synchronous motors: an embedded type permanent magnet motor having an embedded permanent magnet in the rotor, a surface type permanent magnet motor having a permanent magnet attached at the periphery of the rotor, a field winding type synchronous motor having a field winding on the rotor side to draw a current therein to generate a magnetic flux, etc.
All of these synchronous motors have a magnetic flux on the rotor side. There are various methods of having a magnetic flux. For example, a permanent magnet motor includes a permanent magnet on the rotor, and a field winding type motor generates a magnetic flux by sending a current to the field winding. A motor generates torque by drawing a current in a stator winding on the stator side in such a way that the current perpendicularly intersects with the magnetic flux on the rotor side.
As mentioned above, the motor has a magnetic flux; therefore, at high rotational speeds of the motor, an induced voltage occurs according to the rotational speed. The greater the rotational speed of the motor becomes, the higher the induced voltage becomes. If the induced voltage exceeds a system voltage, an adequate current to output a required torque cannot be sent at that rotational speed.
Measures generally taken to restrain the rise of the induced voltage are referred to as “field weakening control.” With a permanent magnet motor, field weakening control is achieved by sending a current in the negative direction of the d axis so as to cancel the magnetic flux generated by the rotor in the magnetic flux direction of the rotor (generally in the d-axis direction) out of the stator winding currents. With a field winding type synchronous motor, on the other hand, the magnetic flux can be reduced by directly reducing the field current.
Further, when a field winding type motor is used, the above-mentioned two field weakening control methods are used together in some applications. In this case, as disclosed in Japanese Patent No. 3331734, there are two different methods. In one method, the field current is sent in inverse proportion to the rotational speed. In the other method, the stator winding current is split into a component having the same phase as the induced voltage and a component perpendicularly intersecting therewith (so-called d-axis current and q-axis current, respectively), and each current is controlled. Under the methods disclosed in Japanese Patent No. 3331734, it is possible to generate a desired torque while suppressing a voltage induced by the motor because each of the field current and the d-axis current can be controlled.    Patent reference 1: Japanese Patent No. 3331734